The Poor Knights marine reserve is at risk from an invasive pest found in Whangarei Harbour, says Northland biosecurity expert Don McKenzie.
The reserve attracts thousands of divers every year and brings an estimated $8 million into the Northland economy.
The Mediterranean fanworm (sabella spallanzanii) and the "invasive, opportunist" Asian kelp (undaria) were found during a survey of three fishing vessel hulls at Port Nikau in Whangarei.
Fanworm was also found on a fishing vessel at Marsden Cove, near the mouth of the harbour.
Overseas, the fanworm is known to form dense colonies dominating natural habitat, fouling structures and excluding native marine life.
Mr McKenzie, Northland Regional Council's biosecurity manager, said discovery of fanworm had triggered an all-out response from agencies involved in the Northland regional pest management strategy - with the hulls being cleaned, fanworm removed from adjacent wharf piles and dive surveys of the surrounds under way.
He said Northland could contain fanworm but efforts were doomed to fail unless regions had a co-ordinated approach to deal with marine pests.
"Northland and Southland have the worst marine pests identified in their regional pest management strategies.
Most other councils don't," he said. Some ports were already "staging posts" for the pests, including Ports of Auckland.
He said the country's marine biosecurity defences had been dangerously diminished by Biosecurity New Zealand being absorbed into a new ministry, the Ministry for Primary Industries.
The national plan for pest management had aimed to address marine pest issues but Biosecurity's profile and key messages had been lost since it was absorbed by the super-ministry.
Jereon Jongejans, who has a Tutukaka dive business, said the ecological value of the Poor Knights could only increase.
"Fortunately for the Poor Knights, being a marine reserve the site is only visited by charter and recreational boats, which are usually pretty clean." Lasting damage to the islands would be devastating for the local economy.